I've never been able to get this to work. Here was the original directions.

Located on the iPhone in the directory:

/private/var/root/Library/Preferences

is a file called: com.apple.mobilemail.plist

Inside this plist is a listing named AutoFetchInterval Change this to read how long you want between checks in minutes. I set mine to 5 minutes and it is working great.

This will not give you an option of 5 minutes under Mail preferences, but make sure you are configured for Manual in Mail preferences and then make the change to the plist. Restart your iPhone and boom, you will be checking mail at your configured interval.

The problem is that it when you set it to manual...it never checks the e-mail. I've followed the directions correctly and also tried it other ways and it doesn't work.

Instead of using the modded file on this site, I just pulled that same file off of my own iPhone. I changed the autofetch value to 5 in plist editor (on a mac) and put it back on my iPhone. Restarted and checked settings. It was set to manual by itself, but it checks every 5 min anyway.